In recent years, GaN compound semiconductor material has received much attention as semiconductor material used for short wavelength light emitting devices. A GaN compound semiconductor is formed on an oxide substrate such as a sapphire single crystal substrate, or Group III-V compound substrates by a metalorganic chemical vapor deposition method (MOCVD method) or a molecular beam epitaxy method (MBE method).
A sapphire single crystal substrate has a lattice constant which differs from the lattice constant of GaN by 10% or more. However, since a nitride semiconductor having excellent properties can be formed by forming on a sapphire single crystal substrate a buffer layer comprising AlN or AlGaN, a sapphire single crystal substrate is widely used. For example, as is shown in FIG. 5, when a sapphire single crystal substrate 1 is used, an n-type GaN semiconductor layer 3, a GaN light emitting layer 4, and a p-type GaN semiconductor layer 5 are formed on the sapphire single crystal substrate 1 in this order. Since a sapphire single crystal substrate 1 is insolent, in general, in a device 20 comprising a sapphire single crystal substrate 1, both a negative electrode 12 formed on the n-type GaN semiconductor layer 3 and a positive electrode 13 formed on a p-type GaN semiconductor layer 5 are positioned on one side of the device 20, as is shown in FIGS. 4 and 5. Examples of a method for extracting light from a device 20 comprising the positive and negative electrodes on one side include a face-up method in which light is extracted from the p-semiconductor side using a transparent electrode such as ITO as a positive electrode, and a flip-chip method in which light is extracted from the sapphire substrate side using a high reflective film such as Ag as a positive electrode.
As is explained above, sapphire single crystal substrates are widely used. However, since sapphire is insolent, a sapphire single crystal substrate has some problems. First of all, in order to form the negative electrode 12, the n-type semiconductor 3 is exposed by etching the light emitting layer 4, as is shown in FIG. 5, therefore, the area of light emitting layer 4 is decreased by the area of the negative electrode 12, and output power decreases. Secondly, since the positive electrode 13 and the negative electrode 12 are positioned on the same side, electrical current flows horizontally, current density is increased locally, and the device 20 is heated. Thirdly, since heat conductivity of a sapphire substrate 1 is low, generated heat is not diffused, and the temperature of the device 20 increases.
In order to solve these problems, a method is used in which a conductive base plate is attached to a device comprising an n-type semiconductor layer, a light emitting layer, and a p-type semiconductor layer which are laminated on a sapphire single crystal substrate in this order, the sapphire single crystal substrate is removed, and then a positive electrode and a negative electrode are positioned on both surfaces of the resulting laminate (For example, Japanese Patent (Granted) Publication No 3511970).
In addition, the conductive base plate is formed by plating, not by attaching (For example, Japanese Unexamined Patent Application, First Publication 2001-274507).
Furthermore, when the conductive base plate is formed by plating, an intermediate layer is formed to improve adhesion between a p-type semiconductor and a plating layer, that is, a p-type semiconductor and a conductive base plate (For example, Japanese Unexamined Patent Application, First Publication 2004-47704).